Method of manufacturing silicon carbide semiconductor substrate and method of manufacturing silicon carbide semiconductor device

ABSTRACT

A step of preparing a silicon carbide substrate (S 11 ), a step of forming a first silicon carbide semiconductor layer on the silicon carbide substrate using a first source material gas (S 12 ), and a step of forming a second silicon carbide semiconductor layer on the first silicon carbide semiconductor layer using a second source material gas (S 13 ) are provided. In the step of forming a first silicon carbide semiconductor layer (S 12 ) and the step of forming a second silicon carbide semiconductor layer (S 13 ), ammonia gas is used as a dopant gas, and the first source material gas has a C/Si ratio of not less than 1.6 and not more than 2.2, the C/Si ratio being the number of carbon atoms to the number of silicon atoms.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.14/424,768, filed Feb. 27, 2015, which is a 371 application ofInternational Application No. PCT/JP2013/083325, filed Dec. 12, 2013,which claims the benefit of Japanese Patent Application No. 2013-022220,filed Feb. 7, 2013.

TECHNICAL FIELD

The present invention relates to methods of manufacturing siliconcarbide semiconductor substrates and methods of manufacturing siliconcarbide semiconductor devices, and more particularly to a method ofmanufacturing a silicon carbide semiconductor substrate capable ofreadily manufacturing a silicon carbide semiconductor substrate having ahigh impurity concentration, and a method of manufacturing a siliconcarbide semiconductor device using this silicon carbide semiconductorsubstrate.

BACKGROUND ART

In recent years, silicon carbide (SiC) has been increasingly employed asa material for a semiconductor device in order to allow a higherbreakdown voltage, lower loss and the like of the semiconductor device.Silicon carbide is a wide band gap semiconductor having a band gap widerthan that of silicon which has been conventionally and widely used as amaterial for a semiconductor device. By employing the silicon carbide asa material for a semiconductor device, therefore, a higher breakdownvoltage, lower on-resistance and the like of the semiconductor devicecan be achieved. A semiconductor device made of silicon carbide is alsoadvantageous in that performance degradation is small when used in ahigh-temperature environment as compared to a semiconductor device madeof silicon.

Since silicon carbide has an extremely low impurity diffusioncoefficient, it is difficult to dope the silicon carbide with animpurity through a thermal diffusion process. Methods of forming anactive region in a silicon carbide material include a method ofimplanting ions into an epitaxial growth layer, and an epitaxial growthmethod involving the addition of impurities using a dopant gas (seeJapanese Patent Laying-Open No. 2002-280573 (PTD 1), for example).

Generally, when forming an n type epitaxial layer on a silicon carbidesubstrate, nitrogen (N₂) gas is used as a dopant gas. A growthtemperature during this process is generally approximately not less than1400° C. and not more than 1700° C.

Nitrogen molecules include a triple bond between nitrogen atoms,however. It is thus difficult to thermally decompose nitrogen moleculesand introduce nitrogen atoms as an active species into the siliconcarbide epitaxial layer.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 2002-280573

SUMMARY OF INVENTION Technical Problem

A possible method of increasing the amount of nitrogen atoms introducedinto a silicon carbide epitaxial layer to thereby increase the impurityconcentration in the silicon carbide epitaxial layer is to reduce aratio of the number of carbon (C) atoms to the number of silicon (Si)atoms (C/Si ratio) in a source material gas used during epitaxialgrowth.

Generally, epitaxial growth of an n type silicon carbide film using N₂gas as a dopant gas is performed under conditions such that the C/Siratio of carbon (C) to silicon (Si) in a source material gas is not lessthan 1.0 and not more than 1.5. This is because, when N₂ gas is used asa dopant gas and the C/Si ratio is higher than 1.5, it is difficult toadd N as an impurity into the epitaxial layer to a sufficient degree.This is also because, when N₂ gas is used as a dopant gas and the C/Siratio is lower than 1.0, although N can be added as an active speciesinto the epitaxial layer to a sufficient degree, it is believed that thegrown epitaxial layer has poor morphology.

Accordingly, if epitaxial growth is performed using a source materialgas having a lower C/Si ratio than the above conventional value so as toincrease the impurity concentration in a silicon carbide epitaxiallayer, it is believed that the silicon carbide epitaxial layer to beobtained has even poorer morphology.

The present invention has been made to solve the problem as describedabove. A main object of the present invention is to provide a method ofmanufacturing a silicon carbide semiconductor substrate capable ofreadily manufacturing a silicon carbide semiconductor substrate whichincludes an n type silicon carbide epitaxial film having a high impurityconcentration and which has good morphology, and a method ofmanufacturing a silicon carbide semiconductor device.

Solution to Problem

The present inventor conducted a detailed research in order to solve theproblem as described above, and found that a silicon carbidesemiconductor substrate which includes an n type silicon carbideepitaxial film having a high impurity concentration and which has goodmorphology can be manufactured by growing an n type silicon carbideepitaxial layer using ammonia (NH₃) gas as a dopant gas, and using asource material gas having a C/Si ratio, which is the number of C atomsto the number of Si atoms, of not less than 1.6 and not more than 2.2.

A method of manufacturing a silicon carbide semiconductor substrate ofthe present invention includes steps of preparing a silicon carbidesubstrate, forming a first silicon carbide semiconductor layer on thesilicon carbide substrate using a first source material gas, and forminga second silicon carbide semiconductor layer on the first siliconcarbide semiconductor layer using a second source material gas, inwhich, in the step of forming a first silicon carbide semiconductorlayer and the step of forming a second silicon carbide semiconductorlayer, ammonia gas is used as a dopant gas, and the first sourcematerial gas has a C/Si ratio of not less than 1.6 and not more than2.2, the C/Si ratio being the number of carbon atoms to the number ofsilicon atoms.

Consequently, the method of manufacturing a silicon carbidesemiconductor substrate of the present invention can readily manufacturea silicon carbide semiconductor substrate which includes an n typesilicon carbide epitaxial film having a high impurity concentration andwhich has good surface morphology.

An impurity concentration in the first silicon carbide semiconductorlayer is higher than an impurity concentration in the second siliconcarbide semiconductor layer. The thickness of the first silicon carbidesemiconductor layer is smaller than the thickness of the second siliconcarbide semiconductor layer.

A method of manufacturing a silicon carbide semiconductor device of thepresent invention includes steps of preparing a silicon carbidesemiconductor substrate, and processing the silicon carbidesemiconductor substrate. In the step of preparing a silicon carbidesemiconductor substrate, the silicon carbide semiconductor substrate ismanufactured with the method of manufacturing a silicon carbidesemiconductor substrate according to the invention described above.

Consequently, performance degradation of the silicon carbidesemiconductor device resulting from defects and poor morphology of thesilicon carbide semiconductor substrate including the n type epitaxialfilm having a high impurity concentration can be suppressed, thusallowing for high-yield manufacturing of silicon carbide semiconductordevices.

Advantageous Effects of Invention

According to the method of manufacturing a silicon carbide semiconductorsubstrate of the present invention, a silicon carbide semiconductorsubstrate which includes an n type silicon carbide epitaxial film havinga high impurity concentration and which has good morphology can bereadily manufactured. Moreover, according to the method of manufacturinga silicon carbide semiconductor device of the present invention,performance degradation of the silicon carbide semiconductor deviceresulting from defects and poor morphology of the silicon carbidesemiconductor substrate can be suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a silicon carbide semiconductor substrateof this embodiment.

FIG. 2 is a flowchart of a method of manufacturing the silicon carbidesemiconductor substrate of this embodiment.

FIG. 3 is a schematic diagram of a vapor phase epitaxy device used inthe method of manufacturing the silicon carbide semiconductor substrateof this embodiment.

FIG. 4 is a flowchart of a method of manufacturing a silicon carbidesemiconductor device of this embodiment.

FIG. 5 shows an image of an example sample of this example, which wasobserved with a differential interference microscope.

FIG. 6 shows an image of a comparative example sample 1 of this example,which was observed with the differential interference microscope.

FIG. 7 shows an image of a comparative example sample 2 of this example,which was observed with the differential interference microscope.

DESCRIPTION OF EMBODIMENTS

A method of manufacturing a silicon carbide semiconductor substrateaccording to an embodiment of the present invention will be describedbelow. The method of manufacturing a silicon carbide semiconductorsubstrate according to this embodiment is a method of manufacturing asilicon carbide semiconductor substrate by stacking a plurality ofsilicon carbide epitaxial layers having different impurityconcentrations on a silicon carbide substrate. Referring first to FIG.1, a silicon carbide semiconductor substrate 10 according to thisembodiment is described. Silicon carbide semiconductor substrate 10according to this embodiment includes a silicon carbide a buffer layer 2made of silicon carbide and formed on silicon carbide substrate 1, and adrift layer 3 made of silicon carbide and formed on buffer layer 2.

Silicon carbide substrate 1 is made of single-crystal silicon carbide,for example. The single-crystal silicon carbide has a hexagonal crystalstructure, for example. Silicon carbide substrate 1 has a main surface1A.

Buffer layer 2 is formed on main surface 1A of silicon carbide substrate1. Buffer layer 2 has n type conductivity and a thickness of 0.5 μm. Ann type impurity concentration in buffer layer 2 is approximately 1×10¹⁸cm⁻³. Buffer layer 2 has a main surface 2A.

Drift layer 3 is formed on buffer layer 2. Drift layer 3 has n typeconductivity and a thickness of 10 μm. An n type impurity concentrationin drift layer 3 is approximately 7×10¹⁵ cm³. Drift layer 3 has a mainsurface 3A, which serves as a main surface of silicon carbidesemiconductor substrate 10.

Referring now to FIGS. 1 and 2, the method of manufacturing a siliconcarbide semiconductor substrate of this embodiment for manufacturing theabove-described silicon carbide semiconductor substrate is described.The method of manufacturing a silicon carbide semiconductor substrateincludes a step of preparing a silicon carbide substrate (S11), a stepof forming a buffer layer on the silicon carbide substrate using a firstsource material gas (S12), and a step of forming a drift layer on thebuffer layer using a second source material gas (S13).

First, in the step (S11), silicon carbide substrate 1 is prepared.Silicon carbide substrate 1 is made of single-crystal silicon carbide.Silicon carbide substrate 1 is disk-shaped and has a thickness of 350μm.

Next, in the step (S12), buffer layer 2 is formed using a vapor phaseepitaxy device on silicon carbide substrate 1 prepared in the previousstep (S11). Referring to FIG. 3, a CVD (Chemical Vapor Deposition)device 100 is used as the vapor phase epitaxy device in this embodiment.In CVD device 100, a substrate holder 11 is surrounded by an inductionheating coil 12, a quartz tube 13, a heat insulating material 14, and aheating element 15. Specifically, heating element 15 has a hollowstructure and forms a reaction chamber therein. Substrate holder 11 isprovided within heating element 15, and formed, for example, such thatmain surface 1A of silicon carbide substrate 1 is flush with the surfaceof the reaction chamber when silicon carbide substrate 1 is placedtherein. Heat insulating material 14 is arranged so as to surround theouter periphery of heating element 15. Quartz tube 13 is arranged so asto surround the outer periphery of heat insulating material 14.Induction heating coil 12 includes a plurality of coil members, and isprovided, for example, so as to be wound around the outer periphery ofquartz tube 13. When a high-frequency current is passed throughinduction heating coil 12 serving as a high-frequency coil, heatingelement 15 is inductively heated by the action of electromagneticinduction. Consequently, silicon carbide substrate 1, the sourcematerial gases supplied to silicon carbide substrate 1, and the like canbe heated to a prescribed temperature.

First, silicon carbide substrate 1 is placed in substrate holder 1provided within CVD device 100. Then, a carrier gas containing hydrogen(H₂), and a source material gas containing monosilane (SiH₄), propane(C₃H₈), ammonia (NH₃) and the like are introduced into CVD device 100through a pipe 16. Here, each gas is introduced into the reactionchamber such that the gas has been thermally decomposed to a sufficientdegree at the time when supplied onto main surface 1A of silicon carbidesubstrate 1. The gases may be mixed together before being introducedinto the reaction chamber of CVD device 100, or may be mixed togetherwithin the reaction chamber of CVD device 100.

Silicon carbide substrate 1 placed on substrate holder 11 receives asupply of the above-described carrier gas and source material gas whilebeing heated, thus causing the formation of buffer layer 2 which is anepitaxial growth film doped with nitrogen (N) atoms on main surface 1A.Specifically, buffer layer 2 is formed under conditions including agrowth temperature of not less than 1500° C. and not more than 1650° C.,and a pressure of not less than 8×10³ Pa and not more than 12×10³ Pa.Here, a flow rate of the NH₃ gas is adjusted such that the n typeimpurity concentration in buffer layer 2 is approximately 1×10¹⁸ cm³.Buffer layer 2 has a thickness of approximately 0.5 μm.

In the first source material gas used to form buffer layer 2 in thisstep (S12), a ratio of the number of C atoms to the number of Si atoms(C/Si ratio) is not less than 1.6 and not more than 2.2. This is becausethe use of a source material gas having a C/Si ratio higher than 2.2results in the occurrence of crystal defects in buffer layer 2 to beformed. This is also because the use of a source material gas having aC/Si ratio lower than 1.6 results in an increase in backgroundconcentration of N atoms in buffer layer 2 to be formed. To the extentthat the background concentration of N atoms is allowed, the surfacemorphology of buffer layer 2 to be formed can be obtained if the C/Siratio is not less than 1.0. A ratio of the number of Si atoms to thenumber of H atoms (Si/H ratio) is not less than 0.0002 and not more than0.0006. A ratio of the number of ammonia molecules to the number ofhydrogen molecules (NH₃/H₂ ratio) is not less than 2.0×10⁻⁸ and not morethan 1.0×10⁻⁶.

Next, in the step (S13), drift layer 3 is formed using the CVD device onbuffer layer 2 formed in the previous step (S12). First, a carrier gascontaining H₂, and a source material gas containing SiH₄, C₃H₈, NH₃ andthe like are introduced into the reaction chamber. Here, each gas isintroduced into the reaction chamber such that the gas has beenthermally decomposed to a sufficient degree at the time when suppliedonto main surface 1A of silicon carbide substrate 1.

Silicon carbide substrate 1 placed in the reaction chamber receives asupply of the above-described carrier gas and source material gas whilebeing heated, thus causing the formation of drift layer 3 which is anepitaxial growth film doped with N atoms on buffer layer 2.Specifically, drift layer 3 is formed under conditions including agrowth temperature of not less than 1500° C. and not more than 1650° C.,and a pressure of not less than 8×10³ Pa and not more than 12×10³ Pa.Here, a flow rate of the NH₃ gas is adjusted such that the n typeimpurity concentration in drift layer 3 is approximately 7×10¹⁵ cm⁻³.Drift layer 3 has a thickness of approximately not less than 10 μm andnot more than 15 μm.

In the second source material gas used to form drift layer 3 in thisstep (S13), the ratio of the number of C atoms to the number of Si atoms(C/Si ratio) is not less than 1.6 and not more than 2.2. This is for thesame reason as that for the first source material gas in the previousstep (S12). The ratio of the number of Si atoms to the number of H atoms(Si/H ratio) is not less than 0.0002 and not more than 0.0006. The ratioof the number of ammonia molecules to the number of hydrogen molecules(NH₃/H₂ ratio) is not less than 2.0×10⁻⁸ and not more than 1.0×10⁻⁶.

After the formation of buffer layer 2 in the previous step (S12) iscompleted, this step (S13) may be performed successively by varying theflow rate and partial pressure of the material gas, with silicon carbidesubstrate 1 remaining on substrate holder 11. In other words, in themethod of manufacturing a silicon carbide semiconductor substrate ofthis embodiment, the first source material gas and the second sourcematerial gas contain the same gaseous species with different flow ratesand partial pressures of the gases. Accordingly, buffer layer 2 anddrift layer 3 can be readily grown successively.

What is particularly important in the method of manufacturing a siliconcarbide semiconductor substrate of this embodiment is the C/Si ratio inthe first source material gas used in the step (S12) and the secondsource material gas used in the step (S13).

When a source material gas having a high C/Si ratio is used, although asilicon carbide epitaxial layer to be formed has good morphology, theamount of N atoms doped into the silicon carbide epitaxial layer islimited. Thus, the use of a source material gas having a high C/Si ratioresults in difficulty in forming a silicon carbide epitaxial layerhaving a high impurity concentration.

When a source material gas having a low C/Si ratio is used, on the otherhand, although a silicon carbide epitaxial layer doped with a higherconcentration of N atoms can be formed, the silicon carbide epitaxiallayer has poorer morphology. Thus, the use of a source material gashaving a low C/Si ratio results in difficulty in forming a siliconcarbide epitaxial layer having good morphology.

In a conventional method of manufacturing a silicon carbidesemiconductor substrate, each silicon carbide epitaxial layer is formedusing N₂ gas as a dopant gas, and using a source material gas having aC/Si ratio of not less than 1.0 and not more than 1.5 regardless of theimpurity concentration. Nitrogen molecules used for the dopant gasinclude a triple bond between nitrogen atoms, however. It is thusdifficult to thermally decompose nitrogen molecules and introducenitrogen atoms as an active species into the silicon carbide epitaxiallayer. It is even more difficult to dope main surface 1A of the siliconcarbide substrate with N atoms evenly within the surface. On the otherhand, the silicon carbide semiconductor substrate to be obtained haspoor morphology and includes numerous defects. When each silicon carbideepitaxial layer is formed using a source material gas having a C/Siratio of not less than 1.0 and not more than 1.5 in the conventionalmethod, although the amount of introduced nitrogen atoms can be up toapproximately 2×10¹⁸ cm³, the silicon carbide epitaxial layer includesnumerous defects.

That is, in the conventional method of manufacturing a silicon carbidesemiconductor substrate using N₂ as a dopant gas, reducing the C/Siratio in the source material gas so as to form an n type silicon carbideepitaxial layer having a higher impurity concentration than the currentconcentration results in even poorer morphology of the silicon carbidesemiconductor substrate to be obtained. Increasing the C/Si ratio in thesource material gas so as to improve the morphology as compared to thecurrent morphology, on the other hand, results in even more difficultyin manufacturing a silicon carbide semiconductor substrate including ann type silicon carbide epitaxial layer having a high impurityconcentration.

In the method of manufacturing a silicon carbide semiconductor substrateof this embodiment, therefore, NH₃ is used as a dopant gas, and a sourcematerial gas having a higher C/Si ratio than in the conventional methodof manufacturing a silicon carbide semiconductor substrate is used.

NH₃ requires a lower temperature than N₂ for thermal decomposition, andis readily decomposed at a general growth temperature when forming asilicon carbide epitaxial layer (approximately between 1400° C. and1700° C. as described above). N atoms are thus readily introduced as anactive species into the silicon carbide epitaxial layer. As a result,even if the C/Si ratio is made higher than in the conventional method ofmanufacturing a silicon carbide semiconductor substrate, a siliconcarbide epitaxial layer having a higher impurity concentration than inthe conventional manufacturing method can be formed. Specifically, evenif the C/Si ratio is not less than 1.6 and not more than 2.2, a siliconcarbide epitaxial layer having a high impurity concentration ofapproximately 2×10¹⁸ cm⁻³ can be formed. Furthermore, since the C/Siratio can be made higher than in the conventional method ofmanufacturing a silicon carbide semiconductor substrate, a siliconcarbide semiconductor substrate having better morphology thanconventional morphology can be fabricated.

Furthermore, as described above, when N₂ gas is used as a dopant gas asin the conventional method of manufacturing a silicon carbidesemiconductor substrate, it is preferable to reduce the C/Si ratio so asto form a silicon carbide epitaxial layer having a high impurityconcentration, however, this results in poorer morphology. For thisreason, it has been difficult in terms of morphology to provide asilicon carbide epitaxial layer having a high impurity concentration asa top layer in a silicon carbide semiconductor substrate. In this case,the poor morphology of the silicon carbide semiconductor substrate needsto be suppressed by providing a silicon carbide epitaxial layer having alow impurity concentration formed with a source material gas having anincreased C/Si ratio on the above-described silicon carbide epitaxiallayer having a high impurity concentration.

In contrast, the method of manufacturing a silicon carbide semiconductorsubstrate of this embodiment can form a silicon carbide epitaxial layerhaving a high impurity concentration using a source material gas havinga high C/Si ratio, thus allowing the silicon carbide epitaxial layer tohave good morphology. According to the method of manufacturing a siliconcarbide semiconductor substrate of this embodiment, therefore, a siliconcarbide semiconductor substrate having an arbitrary configuration can befabricated without imposing limitations in terms of morphology.

Moreover, since the source material gases having the same C/Si ratio canbe used when forming the silicon carbide epitaxial layer having arelatively high impurity concentration and the silicon carbide epitaxiallayer having a relatively low impurity concentration stacked on eachother, they can be grown successively without changing the growthconditions other than the flow rate of the NH₃ gas.

As described above, the method of manufacturing a silicon carbidesemiconductor substrate of this embodiment can readily manufacture asilicon carbide semiconductor substrate which includes a silicon carbideepitaxial layer having a high impurity concentration and which includesfew crystal defects and has good morphology, by using N₂ gas as a dopantgas, and employing the C/Si ratio of not less than 1.6 and not more than2.2 in the source material gases used for growing the silicon carbideepitaxial layers.

Although the silicon carbide semiconductor substrate of this embodimentincludes buffer layer 2 having a thickness of 0.5 μm and an impurityconcentration of 1×10¹⁸ cm⁻³ as a first silicon carbide semiconductorlayer, and drift layer 3 having a thickness of 10 μm and an impurityconcentration of 7×10¹⁵ cm⁻³ as a second silicon carbide semiconductorlayer, the substrate is not limited to include these layers. Forexample, the substrate may include a low impurity concentration layerhaving an impurity concentration of approximately not less than 1×10¹⁴cm⁻³ and not more than 5×10¹⁵ cm⁻³ and a thickness of 20 μm, which isstacked on a high impurity concentration layer having an impurityconcentration of approximately not more than 2×10¹⁸ cm⁻³. Again, withthis configuration, a silicon carbide semiconductor substrate havinggood morphology can be obtained using the above-described sourcematerial gases having the same C/Si ratio.

Although the silicon carbide semiconductor substrate of this embodimenthas a structure in which the two layers having different impurityconcentrations are stacked on the silicon carbide substrate, thesubstrate may have a structure in which three or more layers havingdifferent impurity concentrations are stacked on the silicon carbidesubstrate in an arbitrary configuration. Again, with this configuration,a silicon carbide semiconductor substrate which includes a siliconcarbide epitaxial layer having a high impurity concentration and whichincludes few crystal defects and has good morphology can be obtained, ascompared to a silicon carbide semiconductor substrate obtained with theconventional method of manufacturing a silicon carbide semiconductorsubstrate.

Although the CVD (Chemical Vapor Deposition) device is used as a vaporphase epitaxy device in the method of manufacturing a silicon carbidesemiconductor substrate of this embodiment, the used device is notlimited to this device. Any device capable of forming a silicon carbideepitaxial layer by vapor phase epitaxy can be used.

In the step of forming the silicon carbide epitaxial layer, themorphology can be improved by increasing the growth temperature as well.A high growth temperature of not less than 1700° C. is required in orderto improve the morphology by the growth temperature. Unfortunately, evenif the growth temperature is not less than 1700° C., a silicon carbideepitaxial layer to be formed includes crystal defects to furtherdeteriorate the epitaxial growth device. The method of manufacturing asilicon carbide semiconductor substrate of this embodiment can provide asilicon carbide semiconductor substrate having good morphology withoutincreasing the growth temperature, as compared to the conventionalmethod of manufacturing a silicon carbide semiconductor substrate.

In the method of manufacturing a silicon carbide semiconductor substrateof this embodiment, it is preferable that the growth temperature be notless than 1500° C. and not more than 1650° C. in both the step (S12) andstep (S13). By employing this temperature range, the occurrence ofdefects can be more readily suppressed to provide a silicon carbidesemiconductor substrate having good surface morphology.

Referring now to FIG. 4, the method of manufacturing a silicon carbidesemiconductor device of this embodiment is described. The method ofmanufacturing a silicon carbide semiconductor device of this embodimentincludes a step of preparing a silicon carbide semiconductor substrate(S10) and a step of processing the silicon carbide semiconductorsubstrate (S20).

In the step (S10), a silicon carbide semiconductor substrate ismanufactured with the method of manufacturing a silicon carbidesemiconductor of this embodiment. Consequently, the silicon carbidesemiconductor substrate having good morphology can be prepared.

In the step (S20), the silicon carbide semiconductor substrate preparedin the previous step (S10) is processed to manufacture a silicon carbidesemiconductor device. Specifically, the silicon carbide semiconductorsubstrate is subjected to an ion implantation step, a trench formationstep, a film formation step, an electrode formation step and the like,to manufacture a silicon carbide semiconductor device. Consequently, thesilicon carbide semiconductor device can effectively utilize the siliconcarbide epitaxial layer having a high impurity concentration included inthe silicon carbide semiconductor substrate. In addition, performancedegradation of the silicon carbide semiconductor device resulting fromdefects and poor morphology of the silicon carbide semiconductorsubstrate can be suppressed, thus allowing for high-yield manufacturingof silicon carbide semiconductor devices.

EXAMPLES

Examples of the present invention will be described below.

1. Evaluation Samples (i) Example Sample

First, a silicon carbide substrate having an outer diameter of 4 inchesand a thickness of 350 μm was prepared.

Then, a CVD device was used to grow a silicon carbide epitaxial layer ona main surface of the silicon carbide substrate, and form a buffer layerhaving an impurity concentration of 1×10¹⁸ cm′ to a thickness of 0.5 μm.Here, a carrier gas containing H₂, and a source material gas containingSiH₄, C₃H₈ and NH₃ were introduced into a reaction chamber of the CVDdevice under conditions such that the C/Si ratio is 1.9, Si/H is 0.0004,and NH₃/H₂ is 1×10⁻⁵ cm³. The flow rate of NH₃ was 0.05 sccm. Thepressure in the reaction chamber was not less than 8×10³ Pa and not morethan 12×10³ Pa, and the growth temperature was 1580° C.

Subsequently, the CVD device was used to form a drift layer on a mainsurface of the buffer layer using the same gases and under the samepressure and temperature conditions, with a flow rate of NH₃ of 0.05sccm. The drift layer had an impurity concentration of 7.0×10¹⁵ cm³ anda thickness of 10 μm.

(ii) Comparative Example Sample 1

A comparative example sample 1 basically had the same configuration andwas prepared under the same conditions as the example sample. Thedifference was that the source material gas used to form the bufferlayer and the drift layer was introduced under conditions such that theC/Si ratio was 2.5.

(iii) Comparative Example Sample 2

A comparative example sample 2 basically had the same configuration andwas prepared under the same conditions as the example sample. Thedifference was that the source material gas used to form the bufferlayer and the drift layer was introduced under conditions such that theC/Si ratio was 1.5.

In this manner, three types of silicon carbide semiconductor substrateswere prepared using the source material gas having different C/Siratios.

2. Experiments

The surface morphologies of the three types of silicon carbidesemiconductor substrates thus obtained were evaluated with adifferential interference microscope. Specifically, the surfaces of thesilicon carbide semiconductor substrates were observed with a 10 xobjective lens and a 10 x eyepiece lens. FIG. 5 shows an image of theexample sample observed with the differential interference microscope,FIG. 6 shows an image of comparative example sample 1, and FIG. 7 showsan image of comparative example sample 2.

3. Results

As shown in FIG. 5, the silicon carbide semiconductor substrate of theexample sample prepared at the C/Si ratio of 1.9 had good surfacemorphology. On the other hand, as shown in FIG. 6, etch pits wereidentified on the main surface of the silicon carbide semiconductorsubstrate of comparative example sample 1 prepared at the C/Si ratio of2.5. In addition, as shown in FIG. 7, the silicon carbide semiconductorsubstrate of comparative example sample 2 prepared at the C/Si ratio of1.5 had good surface morphology, however, the silicon carbidesemiconductor substrate of the example sample prepared at the C/Si ratioof 1.9 had better surface morphology. In other words, the siliconcarbide semiconductor substrate prepared at the C/Si ratio of 1.9 hadbetter surface morphology than that of the silicon carbide semiconductorsubstrate prepared at the C/Si ratio of 1.5 or 2.5.

Although the embodiments and examples of the present invention have beendescribed above, the embodiments and examples described above can bemodified in various ways. In addition, the scope of the presentinvention is not limited to the embodiments and examples describedabove. The scope of the present invention is defined by the terms of theclaims, and is intended to include any modifications within the scopeand meaning equivalent to the terms of the claims.

INDUSTRIAL APPLICABILITY

The method of manufacturing a silicon carbide semiconductor substrateand the method of manufacturing a silicon carbide semiconductor deviceof the present invention is applied particularly advantageously to amethod of manufacturing a silicon carbide semiconductor substraterequired to include a silicon carbide epitaxial layer doped with a highconcentration of nitrogen and to have good morphology, and a method ofmanufacturing a silicon carbide semiconductor device.

REFERENCE SIGNS LIST

1 silicon carbide substrate; 1A, 2A, 3A main surface; 2 buffer layer; 3drift layer; 10 silicon carbide semiconductor substrate; 11 substrateholder; 12 induction heating coil; 13 quartz tube; 14 heat insulatingmaterial; 15 heating element; 16 pipe; 10 CVD device.

The invention claimed is:
 1. A method of manufacturing a silicon carbidesemiconductor substrate, comprising steps of: preparing a siliconcarbide substrate; forming a first silicon carbide semiconductor layeron the silicon carbide substrate using a first source material gas; andforming a second silicon carbide semiconductor layer on the firstsilicon carbide semiconductor layer using a second source material gas,wherein an impurity concentration in the first silicon carbidesemiconductor layer is higher than an impurity concentration in thesecond silicon carbide semiconductor layer, in the step of forming afirst silicon carbide semiconductor layer and the step of forming asecond silicon carbide semiconductor layer, ammonia gas being used as adopant gas, the first source material gas having a C/Si ratio of notless than 1.6 and not more than 2.2, the C/Si ratio being the number ofcarbon atoms to the number of silicon atoms, a flow rate of the firstsource material gas being different from a flow rate of the secondsource material gas.
 2. The method of manufacturing a silicon carbidesemiconductor substrate according to claim 1, wherein the thickness ofthe first silicon carbide semiconductor layer is smaller than thethickness of the second silicon carbide semiconductor layer.
 3. Themethod of manufacturing a silicon carbide semiconductor substrateaccording to claim 1, wherein the first source material gas and thesecond source material gas each contain monosilane and propane.
 4. Themethod of manufacturing a silicon carbide semiconductor substrateaccording to claim 1, wherein the impurity concentration in the secondsilicon carbide semiconductor layer is not less than 1×10¹⁴ cm⁻³ and notmore than 7×10¹⁵ cm⁻³.
 5. A method of manufacturing a silicon carbidesemiconductor device, comprising steps of: preparing a silicon carbidesemiconductor substrate; and processing the silicon carbidesemiconductor substrate, in the step of preparing a silicon carbidesemiconductor substrate, the silicon carbide semiconductor substratebeing manufactured with the method of manufacturing a silicon carbidesemiconductor substrate according to claim
 1. 6. A method ofmanufacturing a silicon carbide semiconductor device, comprising stepsof: preparing a silicon carbide semiconductor substrate; and processingthe silicon carbide semiconductor substrate, in the step of preparing asilicon carbide semiconductor substrate, the silicon carbidesemiconductor substrate being manufactured with the method ofmanufacturing a silicon carbide semiconductor substrate according toclaim
 1. 7. A method of manufacturing a silicon carbide semiconductordevice, comprising steps of: preparing a silicon carbide semiconductorsubstrate; and processing the silicon carbide semiconductor substrate,in the step of preparing a silicon carbide semiconductor substrate, thesilicon carbide semiconductor substrate being manufactured with themethod of manufacturing a silicon carbide semiconductor substrateaccording to claim
 2. 8. A method of manufacturing a silicon carbidesemiconductor device, comprising steps of: preparing a silicon carbidesemiconductor substrate; and processing the silicon carbidesemiconductor substrate, in the step of preparing a silicon carbidesemiconductor substrate, the silicon carbide semiconductor substratebeing manufactured with the method of manufacturing a silicon carbidesemiconductor substrate according to claim
 3. 9. A method ofmanufacturing a silicon carbide semiconductor device, comprising stepsof: preparing a silicon carbide semiconductor substrate; and processingthe silicon carbide semiconductor substrate, in the step of preparing asilicon carbide semiconductor substrate, the silicon carbidesemiconductor substrate being manufactured with the method ofmanufacturing a silicon carbide semiconductor substrate according toclaim 4.